1. Field of the Invention
This invention relates to a scanning exposure device wherein a light beam is transmitted upon a flat deflecting device and the reflected light is swept to scan and expose a scanning plane.
2. Description of the Related Art
Generally, a scanning device using laser beam includes a flat deflecting device (e.g., a rotary polygon mirror, etc.) for scanning the light beam, and a focusing lens disposed downstream of the deflecting device. Then, light beams scanned by the flat deflecting device are focused along a plane. A so-called pre-objective type optical system in which a focusing lens is disposed downstream of the deflecting device is known.
In this case, an optical system in which a focusing position in the direction of the optical axis and accuracy of the focused beam position in the scanning direction are taken into account, is demanded. An example is the f.theta. lens which is most commonly used.
The pre-objective type optical system using the f.theta. lens is excellent in focusing position accuracy, uniformity of beam diameter and the like. However, because a large lens and high precision are required, there is a disadvantage that the price is very high. Another idea is to focus a plurality of beams having different wavelengths by means of a single f.theta. lens so as to expose color images. In this case, it is difficult to obtain uniform focusing performance. Depending on the swing angle, the transmission rate differs with wavelength. Depending on this angle, shading, a phenomenon involving tint changes, occurs. Thus, image quality drops.
On the other hand, as a scanning optical system which uses no f.theta. lens, the so-called post-objective type optical system in which the focusing lens is disposed upstream of the deflecting device is known.
This post-objective type optical system is advantageous because its focusing lens is small and so, economical. However, because the focusing position is circular and scanning is performed at angular speed, when a flat type light sensitive material is exposed with the post-objective type optical system, uniformity of beam diameter, accuracy of scanning position, uniformity of scanning speed and other properties are generally inferior to those of the method using the f.theta. lens.
Thus, in the post-objective type optical system, the deflecting device is equipped with curvature or a special lens to correct the focusing characteristic, so as to compensate for the aforementioned deterioration of characteristics.
However, to provide the deflecting device with curvature, a special processing machine and skill for cutting a curved mirror are required, and therefore costs increase.
On the other hand, when a correction lens is interposed, a large non-spherical lens is required to obtain an excellent result, so that an enlargement of the device and a rise in production cost are brought about.